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Wednesday, 14 September 2011

Why do stars live in galaxies?

This post is dedicated to David Allen Green, who asked me after seeing my PubSci talk why the Universe is full of galaxies - stars living in "cities" - rather than stars being evenly spread around, so the Universe is one big galaxy.

People often remark that the Big Bang sounds like an explosion, or is described as an explosion - and that is a violent, chaotic, destructive event, so why did it produce so much order? I have written about the Big Bang itself in more detail here. David knew this was not the case. His question is, in more scientific terms, this: if it produced so much order, and everything expanded - why did it expand with some areas being denser than others?

I'm not going to answer in the traditional way a scientist would expect. I have a tendency to answer things by telling the story in the opposite order from the traditional way. Rather than starting with the Big Bang, I'm going to start with stars.

The Big Bang did not emit stars. It did not even emit atoms. All this stuff came later - when things had cooled and been able to clump. Paradoxically, a star cannot form from hot gas, only cold, because the atoms (or molecules, or ionised atoms and electrons) of a hot gas or plasma are whizzing around too fast to be able to stick together and condense.

So, how can gas cool down? There not being fridges readily available in outer space, it basically needs to be shielded from radiation. This happens in dust clouds. We can't see the centre of our own Milky Way Galaxy because of all the dust in the way. Longer wavelength radiation can get through a lot of it, but not optical (visible light). Where the dust or gas is thick enough, it can cool. And that's when it gets affected by gravity. It contracts.

Star formation typically occurs in clumps, turning the whole area apparently blue. Take a look at these two galaxies and you'll see where the star formation is occurring:

In the spiral galaxy (left), the stars are moving in the same direction. In fact, they move into and out of dense areas, rather like cars moving into and out of traffic jams. This allows regular shock waves to pass through gas clouds, triggering their gravitational collapse and setting off star formation. In the elliptical, on the other hand, each star is going on its own route (see here - click the arrow on the right - for some rather silly star orbits which still remain stable, like a ball falling back to the Earth after being thrown upwards). This leads to the gas being in pretty much a mess, too. There's nowhere it can comfortably clump and cool without being disturbed for a while. Indeed there are no gas clouds left - an elliptical is known for having used up all its gas and having no fuel left. (There is some, but it is too hot and thinly spread to form stars.)

The nearest area to us where star formation like this is occurring now is in Orion's Belt. Next time you see the familiar hunter and those three stars lined up, you can relish the knowledge that although it looks dark around them, there's a churning gas cloud there and a great deal going on inside it - APOD has a gorgeous picture collection here.

Star formation stops in the cluster once the stellar wind from the young stars blows off the rest of the gas; we know the Pleiades are young because there is still a lot of gas surrounding them. Due to the gravity of stars in their local neighbourhoods, these young clusters then tend to drift apart. While in Boston I heard one theory that we may have captured some of our comets and even planets from our sister stars in the Sun's infancy. It was an odd talk . . .

Anyway - this was not the question, but I hope it demonstrates that star formation is not straightforward, and that things need to happen to get it going. I suppose one could say space needs to settle down and get ready.

It does demonstrate why stars don't live outside galaxies: basically, they need to form from gas clouds, and any self-respecting gas cloud that happens to collapse in space won't just generate one star at a time - it'll generate lots! The closest we can get to these is an irregular galaxy. These are clumps of star formation without a local supermassive black hole, and without a defined structure such as spiral or elliptical. They are also far smaller than the big monster we live in and the sort the Zoo has been studying. (This is why Richard's project is so exciting from a purely scientific as well as a citizen science point of view - well, duh, if it wasn't good science, it wouldn't be good citizen science either. But you know what I mean. He has already found that irregular galaxies are much more starforming even than beautiful blue spirals.)

So, stars form when gas clouds collapse. And a good thing too, or we wouldn't be here - not only does the Sun give us light and heat and keep the Earth in a stable orbit, but it's nuclear fusion in stars that creates heavy enough atoms and molecules to form rocks and iron and organic molecules and water and so on that are needed to create life. (As Carl Sagan put it in Pale Blue Dot, it's funny that we consider this the anthropic principle when it might just as well be called "the lithic principle", that the Universe was primed to create rocks, too.)

But why should there be clouds of gas in the first place? If the Big Bang sent everything out in its own direction, and the force of the explosion was equal, sending everything in a sphere (assuming there are three dimensions - in any case, sending equal quantities of everything in equal directions) - then everything should be the same space apart.

Picture a given area of atoms, say of hydrogen. Each is the same weight and has the same gravity. Each is equally spaced from all the others. Each is pulling on the ones around it - so each feels a force from all of its neighbours in every direction. Like a tug of war whose sides are entirely evenly matched, nothing goes anywhere.

But the Universe did not expand quite evenly. Its evenness - its homogeneity - is very, very nearly complete. Especially after inflation. When we look back at the time before any atom was cool enough to get near another, the differences were less than one part in ten thousand.

That time is called "the dark ages" and it's the limit of how far back we can see. There's something in the way. And that's another sort of cloud - or to be exact, a plasma. A plasma is a seething mass of ionised atoms and their electrons - atoms whose electrons have been torn off. (There's probably no net electric charge, since for every negative electron zooming around, there's a positively charged atom somewhere.) The Sun is a plasma. And the one at the edge of the visible Universe is called the Cosmic Microwave Background.

We can't see through it because its edge marks the end of a time in the Universe when it was so hot that light couldn't get through it. (Recall that as you look deep into space, you look back in time. When we look at the Cosmic Microwave Background, we look at a time over 13 billion years ago. When you look at the Sun, you look at a moment 8 minutes ago - and hurt your eyes, incidentally, so I don't recommend that.)

Space was so hot and dense then that whenever a photon of light went anywhere, it promptly collided with an atom or an electron and was sent off elsewhere. It would have been like looking through a thick cloud - or, indeed, the Sun itself, where the same thing happens. (This is why the light that shines down on us is millions of years old. It took that long to escape.) But once the Universe had cooled enough, electrons were able to bind with protons and neutrons, to form neutral atoms. At that point light could get through. We cannot look back any further than that boundary; we have to look at the rest of the Universe and work out what happened before that point.

The COBE satellite, which launched in 1989, made a discovery about the Cosmic Microwave Background that explained our existence: some parts of it were hotter than others. Just a bit. And you've read earlier what hot particles do. They whizz around, they bounce off each other - they don't clump together as easily as cold ones. So everywhere in the Cosmic Microwave Background that was a tiny bit cooler got that tiny bit denser.

And that's where gravity set in. That's where the clouds of hydrogen and helium started to fall together. I have yet to read an astronomy book that doesn't jokingly relate this to capitalism - that the rich get richer and the poor get poorer - in other words, any area with just a bit of density will, over time, attract more and more material. And, conversely, the empty areas get empty. Marcus Chown has written a whole book about how the Cosmic Microwave Background was discovered, and the tiny, tiny fluctuations in it - Afterglow of Creation.

(I once asked Chris if heat alone could account for the fluctuations. Things were very hot then, and as in Brownian motion - the random motion of water molecules that kicked a pollen grain around and therefore allowed Einstein to demonstrate that atoms did exist, and measure their size - there would be a certain amount of randomness: particles moving now one way, now another, like waves on a lake. Would that alone be enough to account for the fluctuations? Chris said no. They were caused by something more, some other irregularity in the Big Bang. I don't know what.)

Stars and galaxies soon formed; the furthest - that is, the earliest we can find - you can read about here. Look at the Hubble Deep Field, a region of space containing vast numbers of very early galaxies, and you'd think that all that uniformity you'd expect from the Big Bang hadn't happened at all.

Before writing this blog post, I dug out Horizons of Cosmology by Joseph Silk, which Astronomy Now had kindly sent me in exchange for a review, and which prompted this blog post. I thought it would probably remind me of a few useful things, and it did.

Galaxies live in clusters. Our own Milky Way does - and it is steadily zooming towards a larger cluster, even while the Milky Way and Andromeda circle each other, ready to merge. And clusters live in superclusters. Superclusters are the largest objects in the Universe. They are like bright filaments through the blackness of space. An accident and emergency doctor and dedicated galaxy classifier once remarked to me that they look remarkably like neurones in the brain.

Silk describes some of the deep sky surveys, the search to understand inflation and the minute differences in temperature that seeded the unevenness, and goes on:

"The larger the region, the more the universe approaches homogeneity. On average, the universe is completely homogenous. There is no dense centre, no rarified boundary region. Yet everywhere there are galaxies. In some regions, there are slightly more than the average, and in others, slightly fewer. We describe these variations as fluctuations in the average density of the Universe. Some are positive, some are negative.

When we measure the strength of the density fluctuations, in other words, we find that the overdensity or underdensity is smaller with increasing scale . . ."

Float away from our world, and look down at it: it will seem huge. Further, and it will shrink, and so too will the Sun, melding into our local group of stars. Later will come our Galaxy's spiral arm, then the galaxy itself. Then the cluster. Then strings of superclusters . . . the further you go, the more you see, the more similarity you will see. It's like when you break the world down to see atoms, and then electrons and quarks. Nature is simple. The Universe is vast. And I love it.

(You may notice I have created a silly new hashtag called Fizzicks Questions. I hope to answer more - and tell you about some good answers I have been given to my own astronomy questions - in the future.)

I allowed this one through as, amazingly, it actually does lead to a post of basic "facts about Earth" (e.g. that the tides are caused by the Moon), though bizarrely we are warned to "ignore these facts at our own peril". I was expecting the usual "I love this post, look at my interesting link which leads to trashy jewellery or viagra adverts", which I delete.